The present invention relates to a novel fiber coupler and a method and apparatus for manufacturing the same and, more particularly, to a fiber coupler having uniform characteristics obtained by manufacturing a plurality of fiber couplers at the same time and a method and apparatus for manufacturing the same.
As optical signal transmission techniques have recently been developed, a number of optical communication systems or optical sensor systems have been proposed and put into practice. General signal transmission techniques aim at high density/high speed signal transmission, stability of characteristics of various elements, and low cost in the optical systems.
An example of systems for realizing high density signal transmission is a fiber ribbon which has been substantially put into practice in recent years. As shown in a sectional view of FIG. 13, this fiber ribbon is obtained by parallelly arranging on a single arrange plane a plurality of bared fibers each of which is formed by coaxially arranging a core 1301, a clad 1302, and a coating 1303, and by surrounding a resultant arrangement by an over coating 1304 to obtain an integral structure. Such a fiber ribbon physically increases density of transmission line per unit sectional area in an optical fiber cable, thereby physically improving transmission density of an optical signal.
When an optical fiber is to be used as a member for transmitting a signal, an optical coupler must be used to branch an optical signal transmitted through a single transmission line into a plurality of transmission lines or to mix two or more optical signals transmitted through a plurality of transmission lines into a single transmission line. An optical coupler having such functions is a member frequently used to construct a variety of optical systems utilizing an optical signal and at the same time a very important member. Especially a fiber coupler using an optical fiber can be easily coupled to an optical fiber which is widely used as an optical transmission line and hence has been used very frequently as various optical transmission systems have been developed.
An example of fiber couplers having a typical arrangement is a biconical tapered single-mode fiber coupler disclosed in Opt. Lett., Vol. 6, 1981, B. S. Kawasaki, K. O. Hill, and R. G. Lamont.
FIGS. 14(a) to 14(c) are views for explaining an arrangement and a manufacturing method of the above conventional fiber coupler. This fiber coupler is manufactured as follows.
That is, as shown in FIG. 14(a), two optical fibers 1401 and 1402 each consisting of only a core 1404 and a clad 1405, i.e., from which a coating is removed, are parallelly arranged to contact each other. Subsequently, as shown in FIG. 14(b), a central portion 1403a of the optical fibers 1401 and 1402 is heated so that the both fibers are fused at this position. Then, when tension is applied to the optical fibers 1401 and 1402 while heating a fused portion to maintain the optical fibers in a softening state, the optical fibers 1401 and 1402 are elongated at the central portion or fused region 1403a, and the cores 1404 move close to each other and hence transmission light leaks therebetween. As a result, an optical fiber as shown in FIG. 14(c) having a fused-elongated region 1403b as a light mixing/branching portion is obtained.
FIG. 15 is a schematic view showing an arrangement of an apparatus for manufacturing the above fiber coupler disclosed in Japanese Patent Application No. 59-88168.
This apparatus fuses and elongates two optical fibers at their central portions to manufacture a fiber coupler. As shown in FIG. 15, the apparatus comprises pairs of fixing tools 1503a-1503b and 1504a-1504b for fixing predetermined sections of optical fibers 1501 and 1502, respectively, and a pair of elongation stages 1505a and 1505b for performing position setting of a fused-elongated region of the optical fiber and giving tension to the optical fiber near the central portion of each of the optical fibers 1501 and 1502.
That is, the fixing tools 1503a-1503b and 1504a-1504b hold the optical fibers 1501 and 1502, respectively, so that the elongation stages 1505a and 1505b can give tension to the optical fibers as will be described below. The elongation stage 1505a has a pair of pins 1506a, and the elongation stage 1505b has a pair of pins 1506b. The pairs of the pins 1506a and 1506b are provided with a predetermined interval therebetween so that the optical fibers 1501 and 1502 are brought into contact with each other at their central portions. The elongation stages 1505a and 1505b comprise horizontal-position adjustment tools 1507a and 1507b movable in a horizontal direction perpendicular to the optical fibers 1501 and 1502, vertical-position adjustment tools 1508a and 1508b which hold the optical fibers 1501 and 1502 and are movable in a vertical direction perpendicular thereto, and temporary fixing tools 1509a and 1509b.
In this apparatus, two optical fibers 1501 and 1502, from which coatings are removed by a predetermined length, are fixed on the fixing tools 1503a-1503b and 1504a-1504b by relatively weak tension, respectively. The optical fibers 1501 and 1502 are held by the elongation stages 1505a and 1505b at substantially a central position between the fixing tools 1503a-1503b and 1504a-1504b. That is, position setting is performed such that the optical fibers 1501 and 1502 are brought into contact with each other in the same plane as that of the surface of the elongation stages 1505a and 1505b by the horizontal-position adjustment tools 1507a and 1507b and that torsion of the optical fibers 1501 and 1502 is prevented by the vertical-position adjustment tools 1508a and 1508b. The optical fibers 1501 and 1502 are fixed in this state by the temporary fixing tools 1509a and 1509b, and the central portions of the optical fibers 1501 and 1502 are fused by a heating tool (not shown). Subsequently, heating is continuously performed to maintain a softening state of the optical fibers 1501 and 1502, and the elongation stages 1505a and 1505b are separated from each other to apply tension to the optical fibers 1501 and 1502, thereby elongating the fibers.
Thus, the fiber coupler as shown in FIG. 14(c) is manufactured.
The above conventional fiber coupler must be manufactured with high accuracy. For example, a force applied on an optical fiber to fix or elongate it is maximally several grams per fiber, and a fused region length of the optical fiber is several millimeters. Factors such as torsion of the optical fibers adversely affect characteristics of the optical fibers. A load or accuracy to such extent is easily varied by friction between members which constitute the apparatus. Therefore, various adjustments must be performed to each optical fiber with high accuracy. However, it is practically impossible to perform such adjustments by only operation of the apparatus. Therefore, in an actual operation, a predetermined amount of light is injected from one end of an optical fiber and monitored at the other end thereof to fuse or elongate the fiber, thereby obtaining uniform characteristics of products.
However, as described above, a large number of fiber couplers are required to construct an optical system. Therefore, when the above operation is performed to each optical fiber, productivity of the fiber couplers is degraded, and manufacturing cost is disadvantageously increased. As a result, practicability of an optical fiber which is expected to take place of a metal cable in the near future is significantly degraded.
In addition, since the fused-elongated region of the fiber coupler is vulnerable to bending, the fused-elongated region must be fixed on a board such as a glass board so that bending or an external force does not act thereon before the fused and elongated fiber coupler is taken out from the fusing/elongating apparatus. However, when this operation is performed to each fiber coupler, productivity of the fiber couplers is degraded. In addition, a large space is required when the optical coupler is mounted in an optical system.
The above problems are significant when a device such as the fiber ribbon described above is used, resulting in not only low productivity of the fiber couplers but also inconvenience. That is, in order to apply the fiber couplers to a system using the fiber ribbon, an operation of manufacturing the fiber couplers in number corresponding to the number of fiber elements to be coupled must be repeatedly performed, and both ends of each fiber coupler must be coupled to cores of the fiber ribbons. In addition, since a slack is required for coupling the optical fibers and each fiber coupler includes a package, a large space must be provided to accommodate the fiber ribbons coupled by the fiber couplers.
Another example of systems in which a plurality of fiber couplers are used is a star coupler as shown in FIG. 16.
An optical fiber type star coupler shown in FIG. 16 is manufactured to branch an optical signal supplied from an input port 1600 equally into eight outputs 1611 to 1618. This coupler is obtained by coupling seven 3-dB fiber couplers 1601 to 1607. As shown in FIG. 16, the input port 1600 is an input port for the coupler 1601. Output ports of the 3-dB fiber coupler 1601 are coupled to next couplers 1602 and 1603 at coupling points 1602a and 1603a, respectively. Output ports of the 3-dB fiber couplers 1602 and 1603 are coupled to input ports of next 3-dB fiber couplers 1604, 1605, 1606, and 1607 at coupling points 1604a, 1605a, 1606a, and 1607a, respectively. The output ports of the 3-dB fiber couplers 1604, 1605, 1606, and 1607 serve as output ports of this star coupler. Note that since an optical signal supplied to the star coupler is attenuated by 3 dB in each 3-dB fiber coupler, light intensity obtained at the output ports 1611 to 1618 is reduced by at least 9 dB.
Since this star coupler is constituted by coupling seven 3-dB fiber couplers 1601 to 1607 by fusing as described above, a connection loss is produced at each of the coupling points 1601a to 1607a. For this reason, not only attenuation of an optical signal in the star coupler is increased but also variations are produced in optical outputs from the output ports 1611 to 1618 due to inevitable variations in connection losses at the coupling points.
In a coupling operation for manufacturing the above star coupler, slack fibers 1601b to 1607b of 30 cm or more must be used before and after the 3-dB fiber couplers 1601 to 1607, respectively. Since the slack fibers 1601b to 1607b remain after the star coupler is completed, a system using such the star coupler cannot be made compact. In addition, a load acting on the coupling points 1601a to 1607a caused by the slack fibers 1601b to 1607b tends to break the coupler near the coupling portion.
As described above, the conventional fiber coupler has poor productivity, operability, and compactness, and hence practical utilization of the fiber coupler to an optical fiber system is largely prevented.